This invention relates generally to ultrasonic motors which convert vibration energy of a piezoelectric element into rotational movement and, particularly, to the mechanical structure of ultrasonic motors suited to stepping operation for use in clocks, watches and their drive methods.
Ultrasonic motors of the prior art generate a progressive or standing wave on an annular or disc-shaped stator by applying a drive voltage on the piezoelectric element and using frictional force to rotate the rotor in contact with teeth formed on the stator. Each of the sub-elements, in which the direction of polarization (poling) is opposite from top to bottom on the element, are alternated and arranged parallel to each other on the piezoelectric element. By shifting the phase of these parallel periods at two locations on the element, two piezoelectric sub-element rows are formed. When an AC voltage is applied on the top and bottom of these piezoelectric sub-element rows, the sub-element rows polarized in the opposite direction are alternately expanded and contracted in the circumferential direction of the piezoelectric element causing overall deformation in the wave shape and generating a progressive wave on the surface of the stator fixed to the piezoelectric element. This progressive wave sequentially changes the height of the teeth formed on the stator and the frictional force between the teeth and the rotor in contact with the teeth cause the rotor to rotate in a prescribed direction.
Such prior art ultrasonic motors present heretofore unresolved problems. When precise positioning control of the rotor is performed such as in stepping operations and in resting at a prescribed rotational position, it is necessary in such devices to attach an encoder or other type of feedback sensor to perform feedback control. Attachment of a position sensor, however, makes difficult construction of the mechanical portion of the assembly compact and such control systems consume significant amounts of power, therefore such devices are particularly unsuited as an ultrasonic motor for clocks and watches requiring a compact, low-power configuration.
This invention is intended to solve these and other problems by providing a compact, low-power ultrasonic motor capable of stepping operation without employing a position sensor and feedback control circuit by using special structures for the rotor and the stator and generating standing waves on the stator.